Level sensor for an elevating platform assembly and elevating platform assembly incorporating the same

ABSTRACT

An elevating platform assembly comprises a pair of laterally spaced, generally vertical masts and an elongated working platform extending generally horizontally between the masts. Hoist mechanisms act between the ends of the working platform the masts and are actuable to move the working platform along the masts to different elevations. At least one level sensor is provided on the working platform and detects when the working platform deviates from a generally horizontal orientation. At least one controller is responsive to the at least one level sensor and controls the hoist mechanisms to adjust movement of the working platform when the working platform deviates from the generally horizontal orientation thereby to return the working platform to the generally horizontal orientation.

FIELD OF THE INVENTION

[0001] The present invention relates generally to elevating platform assemblies and in particular to a level sensor for an elevating platform assembly and an elevating platform assembly incorporating the same.

BACKGROUND OF THE INVENTION

[0002] Elevating platform assemblies having horizontal work floors to support workers and equipment at desired elevations are well known in the art. These elevating platform assemblies allow the elevation of workers and equipment to be changed quickly, offering significant advantageous over their stationary scaffolding counterparts.

[0003] In order to ensure worker and equipment safety, it is desired to maintain the work floors in a generally horizontal orientation and inhibit movement of the work floors when the work floors deviate from this desired orientation beyond a threshold amount.

[0004] For example, PCT International Patent Application No. PCT/SE99/00492 to Alimak A B, published under No. WO 99/50167, discloses a construction hoist system including a pair of spaced masts. Each mast supports a vertical rack and a hoist trolley that is moveable along the mast. The hoist trolley supports a trolley motor and a pinion that is in mating engagement with the rack. The trolley motor drives the pinion to effect vertical movement of the hoist trolley along the mast. A working platform is hingedly coupled at each end to a respective one of the hoist trolleys and provides a work floor to support workers and equipment at desired elevations. A safety device and a level sensor having a pivoting lever arm are mounted on each hoist trolley below the trolley motor. Level sensing devices communicate with the level sensors to control movement of the hoist trolleys when the working platform deviates from a generally horizontal orientation. In particular, when the working platform deviates from a horizontal orientation and pivots about the hinged connections to the hoist trolleys, the lever arms of the level sensors on the hoist trolleys pivot. The level sensors in response to pivoting of the lever arms provide output to the level sensing devices, which in turn control the trolley motors so that further movement of the working platform returns the work floor to a horizontal position.

[0005] PCT International Patent Application No. PCT/SE91/00563 to Alimak A B, published under No. WO 92/06258, discloses a construction hoist system including a pair of spaced masts. Each mast supports a vertical rack and a hoist trolley that is moveable along the mast. The hoist trolley supports a trolley motor and a pinion that is in mating engagement with the rack. The trolley motor drives the pinion to effect vertical movement of the hoist trolley along the mast. A working platform is coupled at each end to a respective one of the hoist trolleys and provides a work floor to support workers and equipment at desired elevations. A leveling mechanism is disposed along the working platform between the hoist trolleys. The leveling mechanism is arranged on the working platform to activate the brakes of the trolley motors if the working platform deviates from a horizontal orientation when the working platform is being lowered manually.

[0006] Although the above references disclose level sensing arrangements for elevating platform assemblies to detect deviations in the orientation of a working platform and to control movement of the working platform, improved level sensing arrangements are desired. It is therefore an object of the present invention to provide a novel level sensor for an elevating platform assembly and an elevating platform assembly incorporating the same.

SUMMARY OF THE INVENTION

[0007] According to one aspect of the present invention there is provided an elevating platform assembly comprising:

[0008] a pair of laterally spaced generally vertical masts;

[0009] an elongated working platform extending generally horizontally between said masts;

[0010] hoist mechanisms acting between opposite ends of said working platform and said masts, said hoist mechanisms being actuable to move said elevating platform along said masts to different elevations;

[0011] at least one level sensor on said working platform and detecting when said working platform deviates from a generally horizontal orientation; and

[0012] at least one controller responsive to said at least one level sensor and controlling said hoist mechanisms to adjust movement of said working platform when said working platform deviates from said generally horizontal orientation thereby to return said working platform to said generally horizontal orientation.

[0013] Preferably, the at least one controller stops the hoist mechanisms when the at least one level sensor detects deviation of the working platform from the generally horizontal orientation beyond a maximum threshold. It is also preferred that the maximum threshold is a 5 degree deviation of the working platform from the horizontal. When the working platform is moving along the masts and deviates from the horizontal by an intermediate threshold that is less than the maximum threshold, the at least one controller adjusts the speed of at least one of the hoist mechanisms relative to the other of the hoist mechanisms to return the working platform to the generally horizontal orientation. In the preferred embodiment, the at least one controller slows one of the hoist mechanisms in response to the at least one level sensor when the working platform is upwardly inclined in a direction away from the mast to which the one hoist mechanism is coupled and the working platform is descending along the mast. The at least one controller speeds up one of the hoist mechanisms in response to the at least one level sensor when the working platform is downwardly inclined in a direction away from the mast to which the one hoist mechanism is coupled and the working platform is ascending the mast.

[0014] According to another aspect of the present invention there is provided a level sensor to detect deviations of a surface from the horizontal comprising:

[0015] a pendulum coupled to said surface and swinging in a generally vertical plane about an arc when said surface deviates from said horizontal; and

[0016] a sensor arrangement detecting swinging movement of said pendulum and generating output signifying deviation of said surface beyond a threshold amount.

[0017] Preferably, the pendulum is disposed between a sensor plate and a pair of stationary proximity sensors. Swinging of the pendulum exposes one or both of the proximity sensors to the sensor plate which causes the one or both proximity sensors to undergo a change in output. One of the proximity sensors is exposed to the sensor plate when the surface deviates from the horizontal beyond an intermediate threshold but below a maximum threshold. Both of the proximity sensors are exposed to the sensor plate when the surface deviates from the horizontal by the maximum threshold.

[0018] Preferably, the level sensor further includes stops to limit movement of the pendulum so that both proximity sensors remain exposed to the sensor plate when the surface deviates from the horizontal beyond the maximum threshold. In the preferred embodiment, the proximity sensors are magnetic proximity sensors, the sensor plate is formed of magnetic material and the pendulum is formed of non-magnetic material.

[0019] According to yet another aspect of the present invention there is provided in an elevating platform assembly including a pair of spaced apart generally vertical masts, an elongated working platform extending generally horizontally between said masts and a hoist mechanism acting between each end of said working platform and a respective mast to move said working platform to different elevations, a method of controlling the hoist mechanisms comprising the steps of:

[0020] monitoring the working platform to detect deviations of said working platform from the horizontal;

[0021] controlling the hoist mechanisms while the working platform is moving to return the working platform to said horizontal when the working platform deviates from the horizontal by at least a first threshold that is less than a maximum threshold; and

[0022] stopping the hoist mechanisms when the working platform deviates from the horizontal by said maximum threshold.

[0023] The present invention provides advantages in that when the working platform is not in a generally horizontal orientation, the hoist mechanisms are controlled in a manner to return the working platform to a generally horizontal orientation. If the orientation of the working platform becomes severe, the hoist mechanisms are stopped to inhibit further movement of the working platform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] An embodiment of the present invention will now be described more fully with reference to the accompanying drawings in which:

[0025]FIG. 1 is a front elevational view of an elevating platform assembly incorporating a level sensor in accordance with the present invention;

[0026]FIG. 2 is an isometric view of a section of the working platform framework that supports the level sensor;

[0027]FIG. 3 is an exploded isometric view of the level sensor; and

[0028]FIGS. 4a to 4 e are front elevational views of a portion of the level sensor for different orientations of the working platform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Referring now to FIG. 1, an elevating platform assembly is shown and is generally identified by reference numeral 10. As can be seen, elevating platform assembly 10 includes two laterally spaced, generally vertical masts 12 a and 12 b respectively, each supported by a base assembly (not shown) that rests on a ground surface. An elongate working platform 16 extends generally horizontally between the masts 12 a and 12 b. The working platform 16 includes a generally planar work surface 20 secured to an underlying supporting framework 22.

[0030] Each end of the working platform 16 is coupled to a respective mast 12 a and 12 b by a hoist mechanism 30 a and 30 b. The hoist mechanisms 30 a and 30 b are actuable to move the working platform 16 vertically along the masts 12 thereby to allow the work surface 20 to be positioned at desired elevations. The hoist mechanisms 30 a and 30 b may be of the type described in U.S. patent application Ser. No. 09/861,864 filed on May 21, 2001, assigned to the assignee of the present invention, the contents of which are incorporated herein by reference. Other types of hoist mechanisms may of course be used.

[0031] An operator control panel 40 is provided on the working platform 16 adjacent mast 12 a and communicates with the hoist mechanisms 30 a and 30 b to allow an operator to control movement of the working platform. The operator control panel 40 is responsive to output received from level sensors 100 a and 100 b mounted on the supporting framework 22 adjacent the masts 12 a and 12 b so that movement of the working platform 16 can be controlled in a manner to return the working platform to a generally horizontal orientation should the working platform 16 deviate from the horizontal by a first threshold amount. The operator control panel 40 also stops movement of the working platform 16 should the working platform 16 deviate from the horizontal by a maximum threshold amount. Further specifics of the manner by which movement of the working platform 16 is controlled will be described herein.

[0032] Turning now to FIG. 2, a section 50 of the supporting framework 22 adjacent mast 12 a is shown. As can be seen, the framework section 50 includes front, intermediate and rear pairs of vertical frame elements 52, 54 and 56 respectively bridged by upper and lower pairs of horizontal frame elements 58 and 60 respectively. The upper horizontal frame elements 58 support the work surface 20. The level sensor 100 a is mounted on the intermediate pair of frame elements 54.

[0033]FIGS. 2 and 3 best illustrate level sensor 100 a. Those of skill in the art will appreciate that level sensor 100 b is the same as level sensor 100 a. As can be seen, level sensor 100 a includes a rectangular boxed-shaped housing 102 having a door 104 hinged to the housing 102. The door 104 is moveable between open and closed positions and can be locked in the closed position to inhibit access to the interior of the housing 102. Vertically spaced lamps 106 a and 106 b are mounted on the door 104 and are illuminated in a manner to provide a visual indication of the orientation of the working platform 16 as will be described. Flanges 108 extend from the top and bottom of the housing 102 and are aligned with upper and lower horizontal supports 62 and 64 bridging the intermediate vertical frame elements 54. Fasteners 110 pass through the flanges 108 and the supports 62 and 64 to secure the level sensor 100 a to the supporting framework 22.

[0034] A support plate 120 is positioned within the housing 102 adjacent its rear wall 122. Spacers 124 adjacent the comers of the support plate 120 secure the support plate 120 to the rear wall 122. A centrally positioned pendulum support post 126 passes through and extends forwardly of the support plate 120 near its upper peripheral edge. The support post 126 passes through a bushing 128 accommodated by a passage in a pendulum 130 and through a spacer 132 disposed between the pendulum 130 and the support plate 120. The support post 126 and the passage in the pendulum 130 are sized to permit the pendulum 130 to swing freely about an arc in a vertical plane. A nut 134 engages the threaded distal end of the support post 126. The spacer 132 and the nut 134 inhibit axial movement of the pendulum 130 along the support post 126.

[0035] The pendulum 130 is in the shape of a sector and is formed of plastic or other suitable non-magnetic material. A pair of inner elongate slots 140 a and 140 b and a pair of outer circular holes 142 a and 142 b are provided through the pendulum 130 adjacent its arcuate end.

[0036] A U-shaped sensor plate 150 formed of magnetic material is secured to the support plate 120 behind the pendulum 130. The sensor plate 150 includes a pair of vertical arms 152 adjacent opposite sides of the support plate 120 that are bridged by a horizontal arm 154. Horizontal arm 154 is positioned and dimensioned so that its forwardly directed planar surface remains in line with the slots and holes provided through the pendulum 130 even as the pendulum swings about support post 126. Retaining blocks 156 a and 156 b are provided on the sensor plate 150 at the intersections between the horizontal and vertical arms 152 and 154 respectively. The retaining blocks 156 a and 156 b present downwardly and outwardly inclined surfaces that face the pendulum 130. The inclined surfaces act as stops to limit pivotal movement of the pendulum 130 as will be described.

[0037] A sensor support 170 spans the retaining blocks 156 a and 156 b and is secured to the retaining blocks by fasteners 172. The sensor support 170 has two horizontally spaced holes 174 therein. The holes 174 receive magnetic proximity sensors 176 a and 176 b . The proximity sensors 176 a and 176 b generate logic output signals that are used by the operator control panel 40 to control movement of the working platform 16.

[0038] During operation of the elevating platform assembly 10, it is desired that the work surface 20 of the working platform 16 be maintained generally in a horizontal orientation for worker and equipment safety. When the working platform 16 is in a horizontal orientation, the pendulum 130 within the level sensor 100 a hangs from the support post 126 in the orientation shown in FIG. 4a. In this orientation, the proximity sensors 176 a and 176 b remain out of alignment with both the slots 140 a and 140 b and the holes 142 a and 142 b provided in the pendulum 130. The ends of the proximity sensors 176 a and 176 b are therefore presented with the non-mettalic surface of the pendulum 130. As a result the outputs of the proximity sensors 176 a and 176 b remain at logic low levels.

[0039] If the elevating platform 16 deviates from the horizontal orientation, the pendulum 130 pivots about the support post 126. The pendulum 130 will pivot in a different direction depending on which end of the working platform 16 becomes raised with respect to the other. For example, if the left-hand end of the working platform 16 as seen in FIG. 1 becomes raised relative to the right-hand end of the working platform, the pendulum 130 pivots about the support post 126 to the right. When the left-hand end of the working platform 16 becomes raised relative to the right-hand end of the working platform by a first threshold equal to three (3) degrees, the pendulum 130 pivots about support post 126 by a sufficient amount to bring the slot 140 b and proximity sensor 176 a into alignment as shown in FIG. 4b. Since the proximity sensor 176 b and the slot 140 b are in alignment, the proximity sensor 176 b detects the metallic surface of the horizontal arm 154 via the slot 140 b in the pendulum 130 and hence, generates a logic high output. The proximity sensor 176 a however, remains out of alignment with the slot 140 a and the hole 142 a and therefore, its output remains at a logic low level.

[0040] If the left-hand end of the working platform 16 becomes raised relative to the right-hand end of the working platform by a maximum threshold equal to five (5) degrees, the pendulum 130 pivots further about support post 126 by a sufficient amount to bring the hole 142 a and proximity sensor 176 a into alignment while maintaining alignment of the slot 140 b and the proximity sensor 176 b as shown in FIG. 4c. Since the proximity sensor 176 b and the slot 140 b as well as the proximity sensor 176 a and the hole 142 a are in alignment, both proximity sensors 176 a and 176 b detect the metallic surface of the horizontal arm 154 and hence, generate logic high outputs. The inclined surface on the retaining block 156 b inhibits the pendulum 130 from pivoting about support post 126 beyond the position shown in FIG. 4c. In this manner even if the left-hand end of the working platform 16 becomes raised relative to the right-hand end of the working platform by more than five (5) degrees, the proximity sensors 176 b and 176 a remain in alignment with the slot 140 b and hole 142 a respectively to maintain the logic high output condition of both proximity sensors 176 a and 176 b.

[0041] Similarly, when the left-hand end of the working platform 16 becomes lowered relative to the right-hand end of the working platform by three (3) degrees, the pendulum 130 pivots about support post 126 by a sufficient amount to bring the slot 140 a and proximity sensor 176 a into alignment as shown in FIG. 4d. Since the proximity sensor 176 a and the slot 140 a are in alignment, the proximity sensor 176 a detects the metallic surface of the horizontal arm 154 via the slot 140 a in the pendulum 130 and hence, generates a logic high output. The proximity sensor 176 b however, remains out of alignment with the slot 140 b and hole 142 b and therefore, its output remains at a logic low level.

[0042] If the left-hand end of the working platform 16 becomes lowered relative to the right-hand end of the working platform by five (5) degrees, the pendulum 130 pivots further about support post 126 by a sufficient amount to bring the hole 140 b and proximity sensor 142 b into alignment while maintaining alignment of the slot 140 a and proximity sensor 176 a as shown in FIG. 4e. Since the proximity sensor 176 a and the slot 142 a as well as the proximity sensor 176 b and hole 142 b are in alignment, both proximity sensors 176 a and 176 b detect the metallic surface of the horizontal arm 154 and hence, generate logic high outputs. The inclined surface on the retaining block 156 a inhibits the pendulum 130 from pivoting beyond the position shown in FIG. 4e. In this manner even if the left-hand end of the working platform 16 becomes lowered relative to the right-hand end of the working platform by more than five (5) degrees, the proximity sensors 176 a and 176 b remain in alignment with the slot 140 a and hole 142 b respectively to maintain the logic high output condition of both proximity sensors 176 a and 176 b.

[0043] The logic output of the proximity sensors 176 a and 176 b within each level sensor 100 a and 100 b is conveyed to the operator control panel 40 and processed. While the logic outputs of the proximity sensors 176 a and 176 b remain at the logic low levels, the elevating platform assembly 10 operates in a conventional manner in response to operator input. However, if the orientation of the working platform 16 deviates sufficiently from the horizontal resulting in a change in the logic output of one or both of the proximity sensors 176 a and 176 b respectively within one or both level sensor 100 a and 100 b, the operator control panel 40 adjusts operation of the hoist mechanisms 30.

[0044] In particular, when the operator control panel 40 receives a logic high signal from proximity sensor 176 b and a logic low signal from proximity sensor 176 a within the level sensor 100 a, signifying that the working platform 16 is declined in a direction away from the mast 12 a, the operator control panel 40 slows the hoist mechanism 30 a if the working platform 16 is moving in an upward direction to allow the right-hand end of the working platform 16 to catch up thereby returning the working platform to a generally horizontal orientation. Otherwise, the operator control panel 40 ignores the proximity sensor 176 b output.

[0045] When the operator control panel 40 receives a logic high signal from proximity sensor 176 a and a logic low signal from proximity sensor 176 b within the level sensor 100 a signifying that the working platform 16 is inclined in a direction away from the mast 12 a, the operator control panel 40 slows the hoist mechanism 30 a if the working platform is moving in a downward direction to allow the right-hand end of the working platform 16 to catch up and return the working platform to a generally horizontal orientation. Otherwise, the operator control panel 40 ignores the proximity sensor output.

[0046] When the operator control panel 40 receives logic high signals from both proximity sensors 176 a and 176 b within the level sensor 100 a, the operator control panel 40 stops the hoist mechanism 30 a since this signifies an unsafe working platform orientation.

[0047] The table below shows the steps performed by the operator control panel 40 in response to the output of the proximity sensors 176 a and 176 b within level sensor 100 a . Elevating Platform Level Sensor Output Orientation Action Proximity sensor 176a Working platform 16 Illuminate lamps 106a logic low output and is generally level. and 106b proximity sensor 176b logic low output Proximity sensor 176a Working platform 16 If working platform 16 is logic high output and is tilted upwards in a moving downwards, slow proximity sensor 176b direction away from hoist mechanism 30a and logic low output mast 12a. illuminate lamp 106a. Proximity sensor 176b Working platform 16 If working platform 16 is logic high output and is tilted downwards in moving upwards, slow proximity sensor 176a a direction away from hoist mechanism 30a and logic low output mast 12a. illuminate lamp 106b. Proximity sensor 176a Working platform 16 Stop hoist mechanisms logic high output and is tilted to an unsafe 30a and 30b. proximity sensor 176a degree. logic high output

[0048] As will be appreciated, the operator control panel 40 controls operation of the hoist mechanism 30 b coupling the right-hand end of the working platform 16 to the mast 12 b in a similar manner based on input received from the proximity sensors 176 a and 176 b of the level sensor 100 b. In this manner, the speed of only one hoist mechanism is varied when the working platform deviates from the horizontal by 3 degrees depending on the direction of movement of the working platform.

[0049] The operator control panel 40 also illuminates the lamps 106 a and 106 b on the doors 104 of the level sensors 100 a and 100 b in a manner to signify visually the orientation of the working platform 16. When the working platform 16 is in a generally horizontal orientation and the outputs of the proximity sensors 176 a and 176 b remain at logic low levels, both lamps 106 a and 106 b are illuminated. If the operator control panel 40 receives a logic high from the proximity sensor 176 b signifying that the working platform 16 is declined in a direction away from the mast 12 a, the bottom lamp 106 b is illuminated. If the operator control panel 40 receives a logic high from the proximity sensor 176 a signifying that the working platform 16 is inclined in a direction away from the mast 12 a, the top lamp 106 a is illuminated. If the operator control panel 40 receives logic highs from both proximity sensors 176 a and 176 b, neither lamp 106 a and 106 b is illuminated. This provides visual feedback to the operator concerning the orientation of the working platform 16.

[0050] In order to maintain smooth operation when the working platform 16 is being raised or lowered, the operator control panel 40 does not immediately react to logic high output received from the proximity sensors 176 a and 176 b. Rather, the operator control panel 40 waits until the logic high output has been maintained for a threshold period of time, before reacting to the logic high output signals. In the preferred embodiment, the threshold period is 0.5 s.

[0051] As will be appreciated, by examining the logic output of the proximity sensors 176 a and 176 b and the order by which the proximity sensors move from logic low to logic high conditions, the orientation of the working platform 16 can be determined. In this manner appropriate action can be taken to control the hoist mechanisms 30 a and 30 b and return the working platform 16 to a generally horizontal orientation.

[0052] Although the elevating platform assembly 10 has been described as including only one operator control panel 40, those of skill in the art will appreciate that an operator control panel may be provided for each hoist mechanism. If an operator control panel is provided adjacent each end of the working platform 16, the logic output of each level sensor 100 a and 100 b is sent to a respective one of the operator control panels. Also, although the elevating platform assembly 10 has been described as including a level sensor 100 a and 100 b adjacent each end of the working platform 16, only one level sensor is required to detect when the working platform 16 is tilted. Furthermore, if desired the speed of both hoist mechanisms can be adjusted to return the working platform to a generally horizontal orientation in response to logic output of the level sensors. In addition, the level sensor can be used on a working platform supported by a single mast to detect deviations of the working platform from the horizontal.

[0053] Although a preferred embodiment of the present invention has been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims. 

What is claimed is:
 1. An elevating platform assembly comprising: a pair of laterally spaced generally vertical masts; an elongated working platform extending generally horizontally between said masts; hoist mechanisms acting between opposite ends of said working platform and said masts, said hoist mechanisms being actuable to move said elevating platform along said masts to different elevations; at least one level sensor on said working platform and detecting when said working platform deviates from a generally horizontal orientation; and at least one controller responsive to said at least one level sensor and controlling said hoist mechanisms to adjust movement of said working platform when said working platform deviates from said generally horizontal orientation thereby to return said working platform to said generally horizontal orientation.
 2. An elevating platform assembly according to claim 1 wherein said at least one controller stops said hoist mechanisms when said at least one level sensor detects deviation of said working platform from said generally horizontal orientation beyond a maximum threshold.
 3. An elevating platform assembly according to claim 2 wherein said maximum threshold is a 5 degree deviation of said working platform from said generally horizontal orientation.
 4. An elevating platform assembly according to claim 2 wherein when said working platform is moving along said masts and deviates from said generally horizontal orientation at least by an intermediate threshold that is less than said maximum threshold, said at least one controller adjusts the speed of at least one of said hoist mechanisms relative to the other of said hoist mechanisms to return said working platform to said generally horizontal orientation.
 5. An elevating platform assembly according to claim 4 wherein said at least one controller adjusts the speed of both of said hoist mechanisms.
 6. An elevating platform assembly according to claim 4 wherein said at least one controller slows one of said hoist mechanisms in response to said at least one level sensor when said working platform is upwardly inclined in a direction away from the mast to which said one hoist mechanism is coupled and said working platform is descending along said masts and wherein said at least one controller speeds up one of said hoist mechanisms in response to said at least one level sensor when said working platform is downwardly inclined in a direction away from the mast to which said one hoist mechanism is coupled and said working platform is ascending said masts.
 7. An elevating platform assembly according to claim 6 wherein said intermediate threshold is a 3 degree deviation of said working platform from said generally horizontal orientation.
 8. An elevating platform assembly according to claim 2 including at least one level sensor on said working platform and one controller controlling operation of both hoist mechanisms.
 9. An elevating platform assembly according to claim 2 including a pair of level sensors and a pair of controllers, each level sensor being positioned on said working platform adjacent a respective one of said masts and being associated with a respective one of said controllers, each of said controllers controlling a respective hoist mechanism in response to said associated level sensor.
 10. An elevating platform assembly according to claim 4 wherein said at least one level sensor includes a pendulum that swings in a generally vertical plane about an arc as said working platform deviates from said generally horizontal orientation, movement of said pendulum being detected thereby to detect said deviation.
 11. An elevating platform assembly according to claim 10 wherein said at least one level sensor includes proximity sensors to detect swinging movement of said pendulum.
 12. An elevating platform assembly according to claim 11 wherein said pendulum is disposed between a sensor plate and a pair of stationary proximity sensors, swinging of said pendulum exposing one or both of said proximity sensors to said sensor plate and causing said one or both proximity sensors to generate output signifying deviation of said working platform from said generally horizontal orientation.
 13. An elevating platform assembly according to claim 12 wherein one of said proximity sensors is exposed to said sensor plate when said working platform deviates from said generally horizontal orientation by said intermediate threshold but below said maximum threshold.
 14. An elevating platform assembly according to claim 13 wherein both of said proximity sensors are exposed to said sensor plate when said working platform deviates from said generally horizontal orientation by said maximum threshold.
 15. An elevating platform assembly according to claim 14 wherein said proximity sensors are exposed to said sensor plate through spaced passages in said pendulum.
 16. An elevating platform assembly according to claim 16 wherein said at least one level sensor further includes stops to limit movement of said pendulum so that both of said proximity sensors remain exposed to said sensor plate when said working platform deviates from said generally horizontal orientation beyond said maximum threshold.
 17. An elevating platform assembly according to claim 16 wherein said proximity sensors are magnetic proximity sensors, said sensor plate is formed of magnetic material and said pendulum is formed of non-magnetic material.
 18. An elevating platform assembly according to claim 4 further including visual indicators providing a visual indication of the orientation of said working platform.
 19. An elevating platform assembly according to claim 18 wherein said visual indicators are carried by said at least one level sensor.
 20. A level sensor to detect deviations of a surface from the horizontal comprising: a pendulum coupled to said surface and swinging in a generally vertical plane about an arc when said surface deviates from said horizontal; and a sensor arrangement detecting swinging movement of said pendulum and generating output signifying deviation of said surface beyond a threshold amount.
 21. A level sensor according to claim 20 wherein said pendulum is disposed between a sensor plate and a pair of stationary proximity sensors, swinging of said pendulum exposing one or both of said proximity sensors to said sensor plate and causing said one or both proximity sensors to generate said output.
 22. A level sensor according to claim 21 wherein one of said proximity sensors is exposed to said sensor plate when said surface deviates from said horizontal by at least an intermediate threshold but below a maximum threshold.
 23. A level sensor according to claim 22 wherein both of said proximity sensors are exposed to said sensor plate when said surface deviates from said horizontal by said maximum threshold.
 24. A level sensor according to claim 23 wherein said proximity sensors are exposed to said sensor plate through spaced passages in said pendulum.
 25. A level sensor according to claim 24 wherein said level sensor further includes stops to limit movement of said pendulum so that said proximity sensors remain exposed to said sensor plate when said surface deviates from said horizontal beyond said maximum threshold.
 26. A level sensor according to claim 25 wherein said proximity sensors are magnetic proximity sensors, said sensor plate is formed of magnetic material and said pendulum is formed of non-magnetic material.
 27. A level sensor according to claim 23 further including visual indicators providing a visual indication of the orientation of said surface.
 28. A level sensor according to claim 27 wherein said pendulum, and sensor arrangement are accommodated within a housing, said visual indicators being provided on a door of said housing.
 29. In an elevating platform assembly including a pair of spaced apart generally vertical masts, an elongated working platform extending generally horizontally between said masts and a hoist mechanism acting between each end of said working platform and a respective mast to move said working platform to different elevations, a method of controlling the hoist mechanisms comprising the steps of: monitoring the working platform to detect deviations of said working platform from the horizontal; controlling the hoist mechanisms while the working platform is moving to return the working platform to said horizontal when the working platform deviates from the horizontal by at least a first threshold that is less than a maximum threshold; and stopping the hoist mechanisms when the working platform deviates from the horizontal by said maximum threshold.
 30. The method of claim 29 wherein said maximum threshold is a 5° deviation of said working platform from said horizontal.
 31. The method of claim 30 wherein said first threshold is a 3° deviation of said working platform from said horizontal. 